Treatment of ballast water carried in ships has become an issue in recent years. Ballast water is seawater carried in a ship to provide safe voyage when the ship is empty of cargo. Ballast water is taken from the waters around a source port, and discharged to the sea during cargo loading at a destination port. Namely, the ballast water taken from the seawater at the source port is discharged at the destination port (cargo loading port). For example, when an oil tanker that departs from Japan sails to the Middle Eastern oil-producing countries such as Kuwait to be loaded with oil, the seawater from Japanese waters is carried as ballast water and discharged to the ocean in Middle Eastern waters. When the ballast water is discharged to a sea area different from the one from which it was taken, marine organisms are transported to the sea area different from their original habitat, significantly affecting the marine ecosystems.
For this reason, a variety of methods of removing, killing or inactivating microbes by purifying ballast water have been developed. For example, Japanese Patent No. 3660984 describes a method of sterilizing underwater organisms by heating seawater, and Japanese Patent No. 4261955 discloses methods using vapor and ultraviolet irradiation. Other methods such as an electrical method using voltage application or shock wave, and a method of injecting an agent such as soda hypochlorite have been proposed. Methods using filtration prior to the killing process, or for the purpose of removing relatively large microbes have also been developed.
For example, Japanese Patent Laying-Open No. 2006-728 describes a method of producing ballast water using a filtration film.